1. Field of the Invention
This invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a semiconductor device including elements partly isolated from each other by a semiconductor region and a method for manufacturing the same.
2. Background Art
In general, element isolation in a semiconductor device is performed by the STI (shallow trench isolation) method, where the silicon substrate is etched to form a trench, in which a dielectric film is buried. However, application of this STI method to an image acquisition device (hereinafter referred to as “imaging device”), such as a CMOS (complementary metal oxide semiconductor) image sensor, causes the following problem.
If photodiodes in the imaging device are isolated from each other by an STI, damage occurs during etching the silicon substrate, and stress is applied during burying a dielectric film in the trench, resulting in introducing crystal defects in the silicon substrate. Thus, the unpaired electron pair of the crystal defect acts as a carrier and produces a white spot in the image. To prevent this, the STI needs to be enclosed by a well (inversion layer), but this decreases the area of the photodiode by the margin of the well. Consequently, the number of saturated electrons during photoelectric conversion decreases, causing degradation in image characteristics, such as decreased sensitivity. This problem manifests itself particularly with the reduction of the pixel pitch.
In this context, as an approach to achieving sufficient element isolation capability while preventing degradation in image characteristics, application of element isolation based on the mesa isolation method to an imaging device is under study. Mesa isolation is a method of PN isolation in which a semiconductor region is formed between elements, and a dielectric film is provided on the semiconductor region. For example, a technique for combined use of STI isolation and mesa isolation in a CMOS image sensor is disclosed in Kazuichiroh Itonaga et al., “A High-Performance and Low-Noise CMOS Image Sensor with an Expanding Photodiode under the Isolation Oxide”, 2005 IEEE 0-7803-9269-8.
However, in the technique disclosed in the above document, the height of the dielectric film of mesa isolation is not matched with the height of the dielectric film of STI. Unfortunately, this increases the difficulty of subsequent processes, which interferes with device downscaling.